Anime Insight: Naruto's Tajū Kage Bunshin no Jutsu

Naruto vs Kimimaro

If I remember it right, I've been watching Naruto since 2004 but the efficiency and danger of Tajū Kage Bunshin no Jutsu or multiple shadow clone technique just dawned on me a while ago. The first time I saw this technique, I wondered what's so special about it? He just created more Kage Bunshin or shadow clones than usual. I don't know why is it even a kinjutsu or a forbidden technique? I thought it's better if every ninja learns about it because it's a great source of man power.